I happens with every game I've tried, but it works fine on WiFi. It was working before it left experimental.
On which device are you playing? If it's unsupported you might need to enable the experimental mode to get it work again.
In this article you can find instructions how to enable it.
If that's not helping, you can also try to reach out to Stadia support to get help or leave feedback in this case via the app if there is no solution yet (feedback helps the Stadia developers to work on issues). To leave feedback just tap your profile picture in the top right corner in the Stadia app or on stadia.google.com and click "feedback".
Thanks for the reply. I think, but can't confirm, that it may have to do with my horrendous up speed on my phone. We just got some security cameras and I can't connect to them either. I'm planning on getting a new phone after Christmas, hopefully that will help.